1) Field of the Invention
The present invention relates to a process for preparing a membranous gas separator which is suitable for use in the separation, adsorption and concentration of gases.
2) Description of the Prior Art
In recent years, the separation of gases by means of a membrane has attracted much attention because the energy required for the separation is low. Among others, oxygen-enriching membranes which enable concentration of atmospheric oxygen are expected to find wide applications in the medical and industrial fields, and a variety of oxygen-enriching polymeric membranes have been proposed. Typically, such oxygen-enriching polymeric membranes are homogeneous membranes. Even when homogeneous membranes have micropores, their average diameter is generally 10 .ANG. or less.
The separation of gases by means of such homogeneous membranes is based on the so-called solution-diffusion mechanism. More specifically, a gas mixture is separated by utilization of the difference in the solubility of gas molecules in the membrane material and of the difference in the diffusion rate of gas molecules in the membrane. A silicone membrance which is a typical polymeric membrane functioning on the basis of this mechanism, has a high permeability coefficient of oxygen, but its O.sub.2 /N.sub.2 separation factor is as low as about 2. Also for membranes formed of other polymers, there is some correlation between the separation factor and the permeability coefficient. Thus, a polymeric material having a higher permeability coefficient tends to give a lower separation factor. Accordingly, the membranes which separate or concentrate gases from their mixture on the basis of this mechanism cannot have both a high permeability coefficient and a high separation factor at the same time.
On the other hand, the separation of gases by means of a porous body, as employed in the enrichment of uranium, is baaed on the utilization of a type of flow called "Knudsen flow". Knudsen flow is a flow in which the flowing gas molecules do not collide with each other and which is encountered in a relatively low pressure range. This type of flow is believed to occur when a gas is allowed to pass through a porous body having a pore diameter less than about one-tenth of the mean free path of the gas molecule at the existing pressure. It is known that a relatively high permeability coefficient can be obtained in the separation of a gas mixture by this mechanism. However, it has been theoretically established that the permeability constant of a gas is inversely proportional to the square root of the molecular weight of the gas molecule. Accordingly, it can be expected that the separation factor will be very low in cases where a mixture of gases having an equal molecular weight or showing a slight difference in molecular weight each other is to be separated. The separation of atmospheric O.sub.2 and N.sub.2 on this principle is not practicable because the theoretical separation factor for these gases is 0.94.
Since 1970, the separation of gases by utilization of a flow arising from the interaction between the pore surfaces of a porous membrane and gas molecules (i.e., anomalous diffusion flow) has been studied. For example, Agrawal and Sourirajan reported the separation of gases by means of a reverse osmosis membrane (i.e., cellulose acetate membrane) [J. Appl. Polymer Sci., 14, 1303-1321 (1970)]. Moreover, Nomi et al. disclosed that, when a membrane having an average pore diameter of 5 nm to 0.1 .mu.m is used, anomalous diffusion flow occurs in specific pressure and temperature ranges (Japanese Patent Publication No. 35402/'81). Both Agrawal et al. and Nomi et al. consider that anomalous diffusion flow is a flow arising from the adsorption of gas molecules to the pore surfaces. Anomalous diffusion flow, when utilized for the separation of a gas mixture, is said to give a higher separation factor than Knudsen flow.
However, the methods of gas separation proposed by Agrawal et al. and Nomi et al. are based on the utilization of anomalous diffusion flow as observed chiefly with hydrocarbons, and do not exhibit satisfactorily high separation efficiency for inorganic gas mixtures (e.g., air). The reason for this is that there has been no suitable porous membrane material capable of interacting with inorganic gas molecules to induce anomalous diffusion.
On the other hand, electrical conductors comprising an electrically conducting polymer laminated to sheet materials or incorporated in film materials are known.
However, in electrical conductors including an electrically conducting polymer, the polymer is usually retained in the form of a layer. This structure itself is not suitable for the separation of gases by utilization of anomalous diffusion flow.
Japanese Patent Laid-Open No. 110729/1987 also discloses a membranous gas separator which retains polypyrrole by electrolytic polymerization in a porous substrate of an organic polymer. Electrolytic polymerization, however, requires complex procedures for connecting the electrode with the porous substrate. When the porous substrate has a large area, it is difficult to conduct polymerization by causing the electric current to flow uniformly over the whole substrate. When a tube having a small diameter is used for the porous substrate, it is difficult to attach the electrode on the inner surface of the tube.
As a result of great efforts to develop a membrane material having a high separation factor for various gas mixtures, the present inventors found that a .pi.-electron conjugated conducting polymer has a unique affinity for gas molecules, particularly O.sub.2 and N.sub.2 molecules (i.e., such a polymer allows O.sub.2 to permeate selectively as compared with N.sub.2 and, moreover, adsorbs O.sub.2 preferentially to N.sub.2), and completed a technique for depositing such a polymer within the pores of a porous substrate. The present invention is based on this technique.